The invention relates to a two circuit resonant bandpass filter for television tuners comprising electronic switches for switching the filter from a first frequency band to a second frequency band.
Such a filter is disclosed in the periodical "IEEE-Trans. CE-22", No. 3, August 1976, pages 203-209, more specifically FIGS. 2 and 3. The prior art bandpass filter is arranged in a tuner between the RF-input stage and a mixer stage. Both the RF-input stage and the mixer stage comprise field effect transistors which do not substantially load the input and the output of the filter. Consequently, the attenuation of the filter is determined by the rather poor quality of the coils and capacitors or capacitance diodes, respectively, used in the filter, as well as by the non-zero forward resistance of the two switching diodes which are used as electronic switches. The primary circuit and the secondary circuit are then damped substantially equally.
Field effect transistors, when used in mixer stages, are, however, comparatively expensive, significantly reduce the frequency swing owing to the parallel arrangement of the gate capacitance (approximately 3 pF) and the resonant circuit capacitance (approximately 4 pF) and, for adequate mixing, require a comparatively large amplitude of the oscillator signals. If, however, a bipolar transistor in a grounded base circuit is used as the mixer stage, then the secondary side of the bandpass filter is subjected to a considerably greater load and, more specifically so, when a coil is arranged between the filter output and the mixer stage input for decoupling purposes. Added to this is the fact that for an optimum decoupling or matching, respectively, the inductance of this coil must have a different value in the higher frequency band than in the lower frequency band. If the coil is divided into two coil sections and if one coil section for the higher frequency band is short-circuited by means of an electronic switch-usually a switching diode-, then the unavoidable capacitance of the electronic switch acts in the lower frequency band in parallel with the coil, which may result in deviations in the frequency variation.
The disadvantages described in the foregoing are particularly noticeable in what are commonly referred to as S-band tuners, that is to say tuners for special channels, of the type which are indispensable for, for example, cable television equipment. The VHF-range I then extends from 48.25 to approximately 108 MHz and the VHF-range III extends from approximately 109 MHz to approximately 300 MHz.
In a known S-band tuner (see Valvo Handbuch "Fernsehtuner, Verzogerungsleitungen" 1978, pages 121-126), a bipolar mixing transistor, which operates in the grounded-base mode, is therefore connected to two RF-input stages via an individual switching diode for each stage, a complete bandpass filter having two circuits for the lower and the upper frequency band, respectively, being arranged between the output of each of the two RF-input stages and the input of the mixer transistor. In contrast to the prior art tuner described in the opening paragraph, the bandpass filter is substantially not attenuated by the switching diodes. The known tuner has, however, the drawback that the number of components is substantially doubled.